PHYSICAL DETERIORATION. 49 



as he is to some extent immune to those diseases. In 

 favour of this belief, the many instances in which a fever 

 has been brought to a country never before accessible to 

 the germ (for instance, the introduction of measles and 

 small-pox to newly discovered America, where fearful 

 ravages were caused thereby), may be brought up as 

 evidence to prove that those habitually living among 

 the germs must have become immune and have trans- 

 mitted this immunity to their progeny. Again, the 

 black population of Sierra Leone have only a mortality 

 of '24 per cent, from malaria, while the mortality of 

 the white settlers is 47 per cent. ; * and, in this case, 

 it may be urged that the black race has become by 

 transmission of immunity partially immune. But these 

 cases which appear to be examples of transmitted im- 

 munity may receive another, and a much more simple, 

 explanation. No two children of the same parents are 

 alike in colour of hair, shape of limb, temperament, etc., 

 and they also differ widely in their capacity to receive 

 and combat infection. An epidemic of fever, therefore, 

 will always select to kill those organically most liable 

 to fall a prey to it, while the remnant, having by 

 nature greater power of resistance, not only survive, 

 but may also be calculated upon to produce progeny, 

 on the whole, as resistant as they are themselves. 



1 Billing's " Text-book of the Theory and Practice of Medi- 

 cine," p. 8. 



